This invention relates to a pneumatic tire, and more particularly, to a pneumatic radial tire which is capable of reducing the ply steer remarkably in pneumatic tires and which has an improved straight driving performance.
The conventional radial tires for cars generally have a construction in which at least two belt reinforcing layers are disposed between a tread and a carcass cord layer substantially parallel to the circumferential direction of the tire, whereby the reinforcing cord of one of the belt reinforcing layers has an angle of 15 to 30 degrees with respect to the tire circumferential direction while the reinforcing cord of the other of the belt reinforcing layers has an angle of 150 to 165 degrees with respect to the tire circumferential direction, the reinforcing cords cross each other, and the carcass cord layer consists of a single or two layers with the cord of each layer describing an angle of about 90.degree. with respect to the tire circumferential direction. When compared with bias tires, the radial tires of this kind have better braking performance, fuel consumption and wear resistance due to the effects brought forth by the belt reinforcing layers, but have the shortcoming that straight driving performance is inferior due to the belt reinforcing layers. Namely, as the radial tire rolls forward, there is a lateral force either to the right or left relative to the travelling direction even if the slip angle is zero, and this lateral force leads the car in a different direction than that intended by the driver.
Generally, the lateral force at the zero slip angle consists of force components that are generated by two different mechanisms. One is referred to as "conicity" (CT) and the other, "ply steer" (PS), and they are classified as parts of the uniformity characteristics of the tire. In accordance with the uniformity test method (JASO C607) for car tires, the conicity CT and the ply steer PS can be expressed by the following equations from their definition in conjunction with LFD, LFDw and LFDs, wherein LFD represents the mean value of the lateral force when the tire rotates once, LFDw is the value measured on the front side and LFDs is the value measured when the tire is turned inside out. EQU LFDw=PS+CT (1) EQU LFDs=PS-CT (2)
PS and CT can be given as follows from the above equations (1) and (2): EQU CT=(LFDw-LFDs)/2 (3) EQU PS=(LFDw+LFDs)/2 (4)
The relation of these equations (1) through (4) can be expressed such as in FIG. 1.
Of the conicity and ply steer, the conicity is believed to result from the fact that the tire shape is geometrically asymmetric about the center of the circumferential direction of the tire, or to be the force that is generated when the tire, which is in the form like a circular truncated cone, rotates. Since the conicity is primarily affected by the positions of the belt reinforcing layers inserted into the tread of the tire, it can be reduced by any improvements made in or relating to the tire production. By contrast, the ply steer is a peculiar force resulting from the structure of the belt reinforcing layers and hence, it has been believed substantially difficult to markedly reduce the ply steer unless the structure itself of the belt reinforcing layers is changed.
Now, the belt reinforcing layer will be discussed in particular. The belt reinforcing layer can be expressed as a two-layered laminate sheet 50 consisting of belt reinforcing layers 50u and 50d, such as depicted in FIG. 2(A). It is well known that when a tensile force is permitted to act upon this two-layered laminate sheet 50 in the tire circumferential direction EE', the two-layered laminate sheet 50 undergoes deformation not only inside the two-dimensional plane in which the tensile force acts, but also three-dimensionally outside the plane, until at last it undergoes torsional deformation as depicted in FIG. 2(B). The ply steer results from this torsional deformation of the belt reinforcing layer.
Various attempts have been made in the past to reduce ply steer by adding a new belt reinforcing layer to the existing belt reinforcing layers, but the addition of a new belt reinforcing layer deteriorates the effects of the radial tire such as low fuel consumption, increases the tire weight, lowers the efficiency and raises the cost of the tire production, and it is not entirely preferable.